No, they will only be used for bonus packs. E.g. if you buy 100 packs with no vouchers, you get 100 packs. If you buy 100 packs and have 10 vouchers, you get the 10% bonus packs and get 110 packs. If you have more than 10 vouchers, but only buy 100 packs, you will only use 10 of those vouchers and still get 110 packs.